Helicopters are an invaluable resource in a wide variety of operational scenarios, both civilian and military. The ability to take off and land effectively vertically is convenient, especially in remote or cramped quarters.
However, the very features that make helicopter operations so invaluable also cause significant challenges. Helicopter pilots are frequently called upon to land or hover over terrain under adverse topographical, topological, meteorological and even situational constraints. Many landings or near landings (in which the helicopter is called upon to hover just above ground level for a considerable period of time while personnel and/or materials are loaded on or off the helicopter) take place on or over rough terrain, such as mountainous regions, building roofs, expanses of soil or sand or even bodies of water. Such operations may be undertaken at night, in winter, and/or during foggy and/or storm conditions.
As a result, it is not uncommon for such landing operations to take place inside an aerosol cloud. For the purposes of the present disclosure, aerosols are small particles suspended in the atmosphere. In the context of helicopter operations, the aerosol may be as a result of prevailing weather phenomena, such as fog or a snow storm, man-made clouds from industrial pollutants, smoke from combusting biomass, or sand, snow, water or dust particles stirred up into a dense cloud by the rotor wash of the helicopter or of a neighboring helicopter.
Under these conditions, pilots cannot see, easily or at all, nearby objects that provide the visual references that enable them to safely control the helicopter near the ground.
In order to ensure safe flight operations, especially during landing and near landing operations, the ability to detect obstacles, such as the terrain features or objects situated thereon, such as trees, animals and humans, vehicles and structures, within an aerosol cloud, is beneficial.
There have been many reported accidents due to brownout and whiteout conditions arising from helicopter rotor wash in both civil and military aviation.
In recent years, the search for a solution has become a high priority for the military, because of the extensive use of helicopter operations in desert regions.
The United States Defense Advanced Research Projects Agency (DARPA) has initiated efforts for many years to develop technologies to assist pilots see through dust during helicopter landings, including the “Sandblaster” program that hopes to test a landing system in September 2008 that promises safer flying for military helicopters in brownout conditions.
Despite efforts by both military and civilian aviation entities, no effective long-term solution to the brownout/whiteout rotor wash problem, or the problem of detecting obstacles within an aerosol cloud generally, has been developed.
It is generally agreed that active sensors, which have the ability of using certain features to discriminate between aerosol and object response provide promising options.
One approach that has been investigated is LIght Detection And Ranging (LIDAR), an optical remote sensing technology that measures properties of scattered collimated light to find range and/or other information of a distant object, typically using laser pulses.
Other approaches include millimeter-wave (MMW) radar and flash LIDAR and range-gated cameras. All of these mechanisms rely on timing discrimination to suppress aerosol signals.
Like radar technology, which uses radio waves instead of light, in LIDAR, an object's range is calculated by measuring the time delay or time of flight (TOF) between transmission of a pulse and detection of a reflection thereof off of the object.
It is theoretically possible to image a feature of an object about the same size as the wavelength of the transmitted pulse, or larger. Because shorter wavelengths in the electromagnetic spectrum are used in LIDAR relative to radar, typically in the ultraviolet, visible or near infrared spectra, LIDAR is highly sensitive to aerosols.
By contrast, the longer wavelength of MMW radar permits it to penetrate deep inside aerosol clouds but provides poor spatial resolution. Additionally, there are significant engineering issues in providing a scanning MMW radar within a compact package.
In a flash LIDAR, an avalanche photo diode (APD) is used to measure TOF and the corresponding distance to a different spot of the object in response to a flash pulse directed at the target, thus providing a 3D ranging solution from a single flash.
While both flash LIDAR and range-gated cameras are able to image the full field of view (FOV) in a single shot and provide high resolution and frame rate, they both lack any substantial ability to penetrate aerosol because the light sources in such devices are spread into the full FOV for each shot.
Scanning LIDAR systems thus pose an optimized trade-off between aerosol penetration and resolution.
The difficulty is in discriminating between returns corresponding to an object and returns corresponding to the surrounding aerosol cloud.
Since aerosols play an important role in environment, they have been investigated by both ground-based LIDARs and space-based LIDARs and the fundamentals of LIDAR response to aerosols have been intensively studied. As a result, the principles of the interaction between LIDAR and aerosols have been well established, for example in studies on pollution monitoring and climate research. However, in such circumstances, the aerosol cloud is relatively extensive, ranging on the order of 1 km in extent, and is of small density.
On the other hand, aerosols generated by helicopter rotor wash, or indeed atmospheric aerosols such as fog and snow/dust storms, are relatively dense and tend to be much more closely localized to the helicopter, typically on the order of 0 m to 100 m. Despite this, the aerosol cloud remains much less dense than an object.